1. Field of the Invention
The present invention relates to a papermaking screen and, more particularly, to a papermaking screen which has a plurality of screens apparatus in a tank thereof.
2. Description of Related Art
Hitherto, there have been papermaking screens which have a plurality of screens in tanks thereof in order to ensure efficient separation of useful fibers and foreign substances (refer, for example, to the papermaking screen disclosed in Japanese Examined Utility Model Publication No. 6-40636 shown in FIG. 10).
In the conventional papermaking screen, a rotary agitator 6' is provided inside an outer cylindrical screen 4' to separate useful fibers and foreign substances while preventing, to a certain extent, the screen from clogging at the same time.
However, positive pressure and negative pressure are applied alternately by the agitator 6' on the papermaking material outlet side of the outer cylindrical screen 4' and therefore, sufficient pressure cannot be produced, causing the outer cylindrical screen 4' to be clogged easily.
Immediately after the agitator 6' passes, the area near an interstice which is composed of a round hole, slit, or the like is subject to negative pressure, causing foreign substances to be drawn toward an acceptor end, adding to the chance of the clogging of the outer cylindrical screen 4'. Furthermore, the agitator 6' rotates inside the outer cylindrical screen 4', so that the moving velocity of the papermaking material slows down when it flows along the outer surface of the outer cylindrical screen 4', presenting a problem of deteriorated dust removing effect. It should be noted that the velocity of papermaking material passing through a interstice which is composed of a round hole, slit, or the like is normally constant, and therefore, the dust removing effect is increased as the moving velocity of the papermaking material flowing along the outer surface of the screen increases. The conventional papermaking screen shown in FIG. 10 has an inlet 1' for a papermaking material, a screened material outlet 2', a tank 3', the outer cylindrical screen 4', a screen basket 5', the agitator 6', an agitator 7', tail discharge ports 8' and 9', a centrifugal cleaner 10', a material pre-screening compartment 11', a material compartment 12', a screening compartment 13', a material supply passage 14', a return passage 15', and a runner 16'.